U.S. patent number 5,660,925 [Application Number 08/568,805] was granted by the patent office on 1997-08-26 for tamper-indicating and authenticating label.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to Henry B. Clark, Julian B. Cooley.
United States Patent |
5,660,925 |
Cooley , et al. |
August 26, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Tamper-indicating and authenticating label
Abstract
A tamper-indicating and authenticating label which provides a
temporary visually perceptible indication that the label is
authentic, and provides a permanent indication of tampering with
the label. The label comprises a normally opaque microporous film
which becomes substantially transparent upon application of a
suitable liquid. The label includes tamper-indicating and
authenticating indicia which are normally not visually perceptible
when the microporous film is opaque, and which become visible when
the microporous film is made transparent. The authenticating
indicia is made temporarily visible to authenticate the label by
applying a volatile liquid to the film to render it transparent,
without dissolving the tamper-indicating indicia. Application of
solvents in an attempt to defeat the adhesive of the label will
cause the tamper-indicating indicia to bleed through to the surface
of the label, providing a permanent indication of tampering.
Application of high temperature in an attempt to defeat the
adhesive of the label will collapse the microvoids of the film,
permanently transparentizing the film and causing the
tamper-indicating indicia be permanently visible. Application of
oil in an attempt to defeat the adhesive of the label will
permanently transparentizing the film and cause the
tamper-indicating indicia be permanently visible. The microporous
film can either be stretchy and permanently deformable, or brittle
and easy to tear or break, to provide additional permanent
indication of tampering.
Inventors: |
Cooley; Julian B. (Hudson,
WI), Clark; Henry B. (Roseville, MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
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Family
ID: |
24272815 |
Appl.
No.: |
08/568,805 |
Filed: |
December 7, 1995 |
Current U.S.
Class: |
428/304.4;
428/195.1; 428/206; 428/321.1 |
Current CPC
Class: |
G09F
3/0292 (20130101); Y10T 428/249995 (20150401); Y10T
428/249953 (20150401); Y10T 428/24893 (20150115); Y10T
428/24802 (20150115) |
Current International
Class: |
G09F
3/02 (20060101); B32B 003/26 () |
Field of
Search: |
;428/195,206,304.4,321.1,30 ;136/156,660 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
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|
|
0 651 365 A1 |
|
May 1995 |
|
EP |
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WO91/04199 |
|
Apr 1991 |
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WO |
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Other References
Bruce N. McBane and Rostyslaw Dowbenko, Pittmentized Coatings Based
on Microporous Films Obtained by Solvent Extraction, Ind. Eng.
Chem., Prod. Res. Develop., vol. 13, No. 1, 1974 (pp.
33-37)..
|
Primary Examiner: Krynski; William
Attorney, Agent or Firm: Griswold; Gary L. Kirn; Walter N.
Trussel; James J.
Government Interests
This invention was made with Government support under a contract
awarded by the Government. The Government has certain rights in the
invention.
Claims
What is claimed is:
1. An authenticatible, tamper-indicating label, comprising:
a normally opaque, transparentizable microporous film having first
and second major surfaces, a first indicia proximate send first
surface a second indicia on said first surface, and an adhesive
proximate said first surface;
wherein said microporous film can be changed from an opaque state
to a transparent state by application of a first liquid that is not
a solvent for said first and second indicia to said microporous
film to thereby sufficiently fill the pores of said microporous
film to cause said film to become transparent;
wherein when said microporous film is in its opaque state, said
first and second indicia are not visually perceptible when said
label is viewed from said second surface, and when said microporous
film is in its transparent state, at least said first indicia is
visually perceptible when said label is viewed from said second
surface, thereby providing an indication of the authenticity of
said label; and
wherein application of a second liquid that is a solvent for said
second indicia causes at least a portion of said second indicia to
migrate through said microporous film to said second major surface,
thereby providing a permanent visually perceptible indication of
tampering.
2. The label of claim 1, wherein said second indicia is soluble in
hydrocarbons.
3. The label of claim 1, wherein said second indicia is soluble in
ketones.
4. The label of claim 1, wherein said second indicia is soluble in
alcohols.
5. The label of claim 1, wherein the application of a volatile
liquid causes said first indicia to be temporarily visually
perceptible through said microporous film, thereby providing a
temporary indication of the authenticity of said label.
6. The label of claim 1, wherein said first indicia is insoluble in
water, such that said first indicia does not migrate through said
microporous film upon application of water to said label.
7. The label of claim 1, wherein said first indicia is insoluble in
trichloroethylene, such that said first indicia does not migrate
through said microporous film upon application of trichloroethylene
to said label.
8. The label of claim 1, wherein said microporous film can be
changed from said opaque state to said transparent state by an
application of heat to a sufficiently high temperature to said
label so as to collapse the pores of said microporous film, thereby
causing said second indicia to be permanently visually perceptible
through said microporous film when viewed from the second surface
to provide a permanent indication of tampering.
9. The label of claim 1, wherein said microporous film comprises a
polymeric film including a plurality of interconnected pores
therein and a plurality of polymeric particles interconnected by
fibrils extending between adjacent particles.
10. The label of claim 9, wherein said polymeric material comprises
polypropylene.
11. The label of claim 9, wherein said polymeric material comprises
polyethylene.
12. The label of claim 9, wherein said polymeric material comprises
ethylene vinyl alcohol copolymer.
13. The label of claim 1, wherein said adhesive includes said
second indicia.
14. The label of claim 1, wherein said microporous film has a
tensile strength of at least 750 psi and exhibits at least 100%
tensile elongation at tensile failure.
15. The label of claim 1, wherein said microporous film has a
tensile strength of at least 1200 psi and exhibits at least 200%
tensile elongation at tensile failure.
16. The label of claim 1, wherein said microporous film has a
tensile strength of no greater than 3500 psi and exhibits no more
than 30% tensile elongation at tensile failure.
17. The label of claim 16, wherein said microporous film has a
tensile strength of no greater than 1000 psi and exhibits no more
than 20% tensile elongation at tensile failure.
18. The label of claim 1, wherein said first indicia is distinct
from said second indicia.
19. An authenticatible, tamper-indicating label, comprising:
a normally opaque, transparentizable microporous film having first
and second major surfaces, a combined indicia proximate said first
major surface, and an adhesive proximate said first surface;
wherein said microporous film can be changed from an opaque state
to a transparent state by application of a first liquid that is not
a solvent for said combined indicia to said microporous film to
thereby sufficiently fill the pores of said microporous film to
cause said film to become transparent; and
wherein when said microporous film is in its opaque state, said
combined indicia is not visually perceptible when said label is
viewed from said second surface, and when said microporous film is
in its transparent state, said combined indicia is visually
perceptible when said label is viewed from said second surface,
thereby providing an indication of the authenticity of said label;
and
wherein application of a second liquid that is a solvent for said
combined indicia causes at least a portion of said combined indicia
to migrate through said microporous film to said second major
surface, thereby providing a permanent visually perceptible
indication of tampering.
20. An authenticatible, tamper indicating system, comprising:
a) an article having an exposed surface;
b) an authenticatible, tamper-indicating label adhered to said
article, said label including a normally opaque, transparentizable
microporous film having first and second major surfaces and an
adhesive, wherein said label is adhered to said exposed surface of
said article with a first bond strength by said adhesive such that
said first major surface is facing said article; and
c) first indicia and second indicia,
wherein said first indicia is located in said system such that when
said microporous film is in its opaque state, said first indicia is
not visually perceptible when said label is viewed from said second
surface, and when said microporous film is in its transparent
state, said first indicia is visually perceptible when said label
is viewed from said second surface, and wherein said second indicia
is located on said first major surface of said film;
wherein said microporous film can be changed from an opaque state
to a transparent state by application of a first liquid that is not
a solvent for said first and second indicia to said microporous
film to thereby sufficiently fill the pores of said microporous
film to cause said film to become transparent;
wherein when said microporous film is in its opaque state, said
first and second indicia are not visually perceptible when said
label is viewed from said second surface, and when said microporous
film is in its transparent state, at least said first indicia is
visually perceptible when said label is viewed from said second
surface, thereby providing an indication of the authenticity of
said label;
wherein application of a second liquid that is a solvent for said
second indicia to said label causes at least a portion of said
second indicia to migrate through said microporous film to said
second major surface, thereby providing a permanent visually
perceptible indication of tampering, and
wherein said microporous film has a tensile strength greater than
said first bond strength, and wherein said microporous film
exhibits at least 100% elongation at tensile failure.
21. The system of claim 20, wherein said microporous film exhibits
at least 200% elongation at tensile failure.
22. An authenticatible, tamper indicating system, comprising:
a) an article having an exposed surface;
b) an authenticatible, tamper-indicating label adhered to said
article, said label including a normally opaque, transparentizable
microporous film having first and second major surfaces and an
adhesive, wherein said label is adhered to said exposed surface of
said article with a first bond strength by said adhesive such that
said first major surface is facing said article; and
c) first indicia and second indicia, wherein said first indicia is
located in said system such that when said microporous film is in
its opaque state, said first indicia is not visually perceptible
when said label is viewed from said second surface, and when said
microporous film is in its transparent state, said first indicia is
visually perceptible when said label is viewed from said second
surface, and wherein second indicia is located on said first major
surface of said film;
wherein said microporous film can be changed from an opaque state
to a transparent state by application of a first liquid that is not
a solvent for said first and second indicia to said microporous
film to thereby sufficiently fill the pores of said microporous
film to cause said film to become transparent;
wherein when said microporous film is in its opaque state, said
first and second indicia are not visually perceptible when said
label is viewed from said second surface, and when said microporous
film is in its transparent state, at least said first indicia is
visually perceptible when said label is viewed from said second
surface, thereby providing an indication of the authenticity of
said label;
wherein application of a second liquid that is a solvent for said
second indicia to said label causes at least a portion of said
second indicia to migrate through said microporous film to said
second major surface, thereby providing a permanent visually
perceptible indication of tampering; and
wherein said microporous film has a tensile strength less than said
first bond strength, and wherein said microporous film exhibits no
more than 30% elongation at tensile failure.
23. The system of claim 22, wherein said microporous film exhibits
no more than 20% elongation at tensile failure.
Description
TECHNICAL FIELD
The present invention relates to a tamper-indicating and
authenticating label which provides a temporary visually
perceptible indication that the label is authentic, and provides a
permanent indication of tampering with the label. The label
comprises a normally opaque microporous film which becomes
substantially transparent upon application of a suitable liquid.
The label includes both tamper-indicating and authenticating
indicia which are normally not visually perceptible when the
microporous film is opaque, and which become visible when the
microporous film is made transparent.
BACKGROUND OF THE INVENTION
Microporous films or membranes have a structure that enables fluids
to flow through them. The effective pore size is at least several
times the mean free path of the flowing molecules, namely from
several micrometers down to about 100 Angstroms. Such films are
generally opaque, even when made of a transparent material, because
the surfaces and the internal structure scatter visible light. When
the pores are filled with a material, typically a liquid, having an
index of refraction sufficiently close to that of the film
material, the film can be made substantially transparent. Upon
removal of the liquid, such as by evaporation, the microporous film
again becomes opaque. This property of microporous films is well
known in the art. Bruce N. McBane and Rostyslaw Dowbenko,
Pittmentized Coatings Based on Microporous Films Obtained by
Solvent Extraction, Ind. Eng. Chem. Prod. Res. Develop., Vol. 13,
No. 1, at 33-37 (1974).
It has been recognized that indicia can be hidden by an opaque
microporous film, and that the indicia can be rendered visible by
wetting the microporous film out with a liquid to fill the
micropores and rendering the film transparent. U.S. Pat. No.
4,846,095 "Critical Temperature Indicating Device" (Emslander)
teaches a device for indicating a critical temperature. A
microporous sheet is exposed to a mixture comprising two liquids
which is incapable of wetting out the film at a given temperature,
but capable of wetting the film at a critical temperature. At the
critical temperature, the components of the mixture separate,
allowing one of the liquids to irreversibly wet the film. Emslander
does not suggest the use of the film in a security label, or the
ability to provide both a temporary indication of authenticity and
a permanent indication of tampering.
It has been recognized in U.S. Pat. No. 4,374,889, "Oil-Repellent
Microvoid-Imaging Material" (Arens), that when a sheet material
having a transparentizable opaque microvoid containing layer is
transparentized with a relatively non-volatile material such as
oil, a permanent mark may remain. Arens teaches a dark-colored
substrate having a microvoid containing layer on both the front
surface (working surface) and back surface. The front surface is
treated with oleophobic fluorocarbons to prevent oil from
penetrating and transparentizing the front surface. The back
surface of an adjoining sheet can be contacted against the front
surface to blot away oil from the from surface to prevent permanent
marks. Arens does not suggest the use of microvoid containing
layers in a security label adhered to an article. Furthermore,
Arens teaches how to prevent permanent marks caused by oil, rather
than providing a permanent indication of tampering in response to
the application of oil.
It is known that a normally opaque microporous film can be rendered
transparent by the application of a sufficiently high temperature
to the film. The high temperature causes the microvoids to
substantially collapse, reducing the internal reflection of light
caused by the microvoids. It has been recognized that this property
of microporous films can be used to provide an indication of
tampering by the application of heat. U.S. Pat. No. 4,733,786,
"Container and Innerseal Capable of Indicating Heat Tampering"
(Emslander) discloses an innerseal for bottles or other containers
comprising a facing layer for adhering to a container opening, an
insulating layer adhered to the facing layer, and a thermally
sensitive layer adhered to the insulating layer. In one embodiment,
the thermally sensitive layer is normally opaque, and becomes
substantially transparent upon application of heat to the
innerseal. When the thermally sensitive layer is transparent,
messages which have been printed on the facing layer, the
insulating layer, or the side of the thermally sensitive layer
adjacent the insulating layer become visible. The message is
normally not visible when the thermally sensitive layer is opaque.
The message provides an indication of heat tampering when the
thermally sensitive layer is transparent. One preferred thermally
sensitive layer taught by Emslander is the microporous sheet
material disclosed in U.S. Pat. No. 4,539,256 "Microporous Sheet
Material, Method of Making and Articles Made Therewith," (Shipman).
However, Emslander does not suggest a security label which is
temporarily transparentized with a liquid to authenticate the
label, nor does Emslander suggest using a soluble tamper-indicating
indicia which permanently bleeds through the thermally sensitive
layer upon tampering with a solvent.
U.S. Pat. No. 5,389,426, "Article for Use in Forming a Permanent
Image Using a Temporary Marker" (Arens et al.), discloses a porous
material having a top surface and a bottom surface, and a dye
applied to the bottom surface of the porous material. The dye is
soluble in a volatile imaging liquid, such that when the volatile
liquid contacts the top surface of the porous material, the
volatile liquid penetrates the material, dissolves at least some of
the dye to provide a dye solution, with at least some of the dye
solution migrating to the top surface. When the volatile liquid
evaporates, a permanently visible mark of dye residue remains on
the top surface of the porous material. The dye may be of any
composition, provided that the dye is soluble in the imaging
liquid. Arens et al. discloses that the porous material may be of
any construction provided the porosity permits the imaging liquid
to penetrate the porous material from the top surface and reach the
dye below. In one embodiment, Arens et al. teaches that the porous
material is preferably any type of porous paper such as Mead 20 lb.
coated front paper, NCR 20 lb. coated paper, or standard 50 lb.
offset paper. In another embodiment, Arens et al. teaches that the
transparentizable microporous layer comprises particles having an
index of refraction from about 1.3 to 2.2 incorporated in a binder
which has an index in the same range as the particles, with
interconnected microvoids being present throughout the layer. Arens
et at. also teaches that in embodiments in which the porous
material is rendered transparent by the volatile liquid, there will
be a visible indication that is a combination of two components.
There will be a temporary component caused by rendering the porous
material transparent allowing the dye to be viewed through the
material, and after a sufficient amount of time, a permanent
component caused by dye which has migrated to the top surface. Upon
evaporation of the liquid, the porous material will no longer be
transparent, and only the permanent component of the indication
will remain visible. However, Arens et at. does not teach applying
two indicia to the bottom surface of the material, one to provide a
temporary indication which does not bleed through the material upon
application of a liquid which does not cause the second indicia to
bleed through, and a second which will provide a permanent
indication by bleeding through the material upon application of
selected solvents.
Thus, it is seen that there is no suggestion in the art to provide
a tamper-indicating and authenticating label which provides a
temporary visually perceptible indication that the label is
authentic, and provides a permanent indication of tampering with
the label, the label comprising a normally opaque microporous film
which becomes substantially transparent upon application of a
suitable liquid and which includes tamper-indicating and
authenticating indicia which are normally not visually perceptible
when the microporous film is opaque, and which become visible when
the microporous film is made transparent.
SUMMARY OF THE INVENTION
The inventive tamper-indicating and authenticating label of the
present invention comprises a microporous film, authenticating
indicia, and tamper-indicating indicia, and provides:
a) a temporary visually perceptible indication that the label is
authentic by applying a volatile liquid to the film to render it
temporarily transparent, without dissolving the tamper-indicating
indicia; and
b) a permanent visually perceptible indication of tampering with
the label caused by any one or a combination of four methods of
tampering:
i) application of solvents in an attempt to defeat the adhesive of
the label will cause the tamper-indicating indicia to bleed through
to the surface of the label, providing a permanent indication of
tampering;
ii) application of high temperature in an attempt to defeat the
adhesive of the label will collapse the microvoids of the film,
permanently transparentizing the film and causing the
tamper-indicating indicia be permanently visible;
iii) application of oil in an attempt to defeat the adhesive of the
label will permanently transparentize the film and cause the
tamper-indicating indicia be permanently visible, and may also
cause the tamper-indicating indicia to bleed through to the
surface;
iv) mechanical pulling or peeling the label to remove it from an
article is indicated by forming the microporous film to be either
stretchy and permanently deformable, or brittle and easy to tear or
break.
One aspect of the present invention presents an authenticatible,
tamper-indicating label comprising a normally opaque,
transparentizable microporous film having first and second major
surfaces, a first indicia, a second indicia proximate the first
surface, and an adhesive proximate the first surface. The
microporous film can be changed from an opaque state to a
transparent state by application of a first liquid that is not a
solvent for the first and second indicia to the microporous film to
thereby sufficiently fill the pores of the microporous film to
cause the film to become transparent. When the microporous film is
in its opaque state, the first and second indicia are not visually
perceptible when the label is viewed from the second surface. When
the microporous film is in its transparent state, at least the
first indicia is visually perceptible when the label is viewed from
the second surface, thereby providing an indication of the
authenticity of the label. Application of a second liquid that is a
solvent for the second indicia causes at least a portion of the
second indicia to migrate through the microporous film to the
second major surface, thereby providing a permanent visually
perceptible indication of tampering. The first and second indicia
can be distinct from one another, or there can be a combined
indicia that serves as both the authenticating indicia and the
tamper-indicating indicia.
In various embodiments of the above label, the second indicia is
soluble in hydrocarbons, ketones, or alcohols, and the first
indicia is insoluble in water or trichloroethylene.
The microporous film of the above label can comprise a polymeric
film including a plurality of interconnected pores therein and a
plurality of polymeric particles interconnected by fibrils
extending between adjacent particles. The polymeric material can
comprise, for example, polypropylene, polyethylene, or ethylene
vinyl alcohol copolymer.
The application of a volatile liquid to the above label causes the
first indicia to be temporarily visually perceptible through the
microporous film, thereby providing a temporary indication of the
authenticity of the label. The application of a sufficiently high
temperature to the label so as to collapse the pores of the
microporous film permanently changes the film from said opaque
state to the transparent state, thereby causing the second indicia
to be permanently visually perceptible through the microporous to
provide a permanent indication of tampering.
To provide an easily deformable film in the above label, in one
preferred embodiment, the microporous film has a tensile strength
of at least 750 psi and exhibits at least 100% tensile elongation
at tensile failure. In another embodiment, the microporous film has
a tensile strength of at least 1200 psi and exhibits at least 200%
tensile elongation at tensile failure. To provide a brittle, easily
torn label, the microporous film has a tensile strength of no
greater than 3500 psi and exhibits no more than 30% tensile
elongation at tensile failure. In another embodiment, the
microporous film has a tensile strength of no greater than 1000 psi
and exhibits no more than 20% tensile elongation at tensile
failure.
Another aspect of the present invention presents an
authenticatible, tamper indicating system, comprising an article
having an exposed surface; an authenticatible, tamper-indicating
label adhered to the article, the label including a normally
opaque, transparentizable microporous film having first and second
major surfaces and an adhesive, wherein the label is adhered to the
exposed surface of the article with a first bond strength by the
adhesive such that the first major surface is facing the article;
and first and second indicia. The microporous film can be changed
from an opaque state to a transparent state by application of a
first liquid that is not a solvent for the first and second indicia
to the microporous film to thereby sufficiently fill the pores of
the microporous film to cause the film to become transparent. When
the microporous film is in its opaque state, the first and second
indicia are not visually perceptible when the label is viewed from
the second surface, and when the microporous film is in its
transparent state, at least the first indicia is visually
perceptible when the label is viewed from the second surface,
thereby providing an indication of the authenticity of the label.
Application of a second liquid that is a solvent for the second
indicia to the label causes at least a portion of the second
indicia to migrate through the microporous film to the second major
surface, thereby providing a permanent visually perceptible
indication of tampering.
In one preferred embodiment of the system, the microporous film has
a tensile strength greater than the first bond strength and the
microporous film exhibits at least 100% elongation at tensile
failure. In a variation of this aspect, the microporous film
exhibits at least 200% elongation at tensile failure.
In another preferred embodiment of the system, the microporous film
has a tensile strength less than the first bond strength and the
microporous film exhibits no more than 30% elongation at tensile
failure. In a variation of this aspect, the microporous film
exhibits no more than 20% elongation at tensile failure.
Certain terms are used in the description and the claims that,
while for the most part are well known, may require some
explanation. It should be understood that, when referring to the
microporous film as being "opaque," this means that the film is
either completely opaque, or sufficiently opaque to prevent the
indicia on the back of the film from being visually perceptible
under ordinary viewing conditions. It should be understood that,
when referring to the microporous film as being as being
"transparent," this means that the film is completely transparent,
or sufficiently translucent to allow indicia on the back of the
film to be readily viewed through the film under ordinary viewing
conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be further explained with reference to
the appended Figures, wherein like structure is referred to by like
numerals throughout the several views, and wherein:
FIG. 1 is a cross-sectional view of a tamper indicating and
authenticating label according to the present invention;
FIG. 2 is a cross-sectional view of the label of FIG. 1 adhered to
an article;
FIG. 3 is a plan view of the label of FIG. 1, in which first and
second indicia are not visually perceptible, and a third indicia is
visually perceptible;
FIG. 4 is a plan view of the label of FIG. 1, in which first,
second, and third indicia are visually perceptible; and
FIG. 5 is a plan view of the label of FIG. 1, in which the first
indicia is not visually perceptible, and in which the second and
third indicia are visually perceptible.
DETAILED DESCRIPTION OF THE INVENTION
Refer now to FIG. 1, in which there is illustrated a
cross-sectional view of a tamper-indicating and authenticating
label 10 according to the present invention. Label 10 comprises a
microporous film 12 having first major surface 14 and second major
surface 16. On first major surface 14 is a suitable means for
adhering the label 10 to an article. As illustrated in FIG. 1, a
layer of adhesive 24 is present on first major surface 14 of the
microporous film 12. An optional release liner 26 may be present
over adhesive 24 as is well known in the art and is preferred when
adhesive 24 is a pressure sensitive adhesive to prevent the label
from inadvertently adhering to any object until the liner 26 is
removed.
Adhesives useful in the preparation of an adhesive coated
labelstock according to the present invention include both pressure
sensitive and non-pressure sensitive adhesives, such as hot melt
and cured adhesives. Pressure sensitive adhesives are normally
tacky at room temperature and can be adhered to a surface by
application of, at most, light finger pressure, while non-pressure
sensitive adhesives include solvent, heat, or radiation activated
adhesive systems. Pressure sensitive adhesives are a preferred
class of adhesives for use in the present invention. Examples of
adhesives useful in the invention include those based on general
compositions of polyacrylate; polyvinyl ether; diene-containing
rubber such as natural rubber, polyisoprene, and polyisobutylene;
polychloroprene; butyl rubber; butadiene-acrylonitrile polymer;
thermoplastic elastomer; block copolymers such as styrene-isoprene
and styrene-isoprene-styrene block copolymers,
ethylene-propylene-diene polymers, and styrene-butadiene polymer;
poly-alpha-olefin; amorphous polyolefin; silicone;
ethylene-containing copolymer such as ethylene vinyl acetate,
ethylacrylate, and ethyl methacrylate; polyurethane; polyamide;
epoxy; polyvinylpyrrolidone and vinylpyrrolidone copolymers;
polyesters; and mixtures of the above. Additionally, the adhesives
can contain additives such as tackifiers, plasticizers, fillers,
antioxidants, stabilizers, pigments, diffusing particles,
curatives, and solvents.
A general description of useful pressure sensitive adhesives may be
found in Encyclopedia of Polymer Science and Engineering, Vol. 13,
Wiley-Interscience Publishers (New York, 1988). Additional
description of useful pressure sensitive adhesives may be found in
Encyclopedia of Polymer Science and Technology, Vol. 1,
Interscience Publishers (New York, 1964).
Other pressure sensitive adhesives useful in the invention are
described in the patent literature. Examples of these patents
include U.S. Pat. No. Re.24,906 (Ulrich); U.S. Pat. No. 3,389,827
(Abere et al.) at Col. 4-Col. 5; U.S. Pat. No. 4,080,348 (Korpman);
U.S. Pat. No. 4,136,071 (Korpman); U.S. Pat. No. 4,181,752 (Martens
et al.); U.S. Pat. No. 4,792,584 (Shiraki et al.); U.S. Pat. No.
4,833,179 (Young et al.); and U.S. Pat. No. 4,952,650 (Young et
al.). Commercially available adhesives are also useful in the
invention. Examples include those adhesives available from 3M
Company, St. Paul, Minn.; H.B. Fuller Company, St. Paul, Minn.; and
Century Adhesives Corporation, Columbus, Ohio.
As illustrated in FIG. 2, label 10 can be adhered to article 28,
thus providing a system comprising an article 28 in combination
with the tamper-indicating, authenticatible label 10. Article 28
can be any article for which there is a desire to determine if
there has been tampering. For instance, article 28 can be an
envelope, package, or other container, to which label 10 has been
adhered across two components 28a and 28b movable relative to one
another, such as the closure flap and body of a container. In such
an arrangement, a user will be able to determine whether the label
10 has been tampered with or removed to gain access to the article
as described below. It is also possible to incorporate label 10 as
a component of such an article. Other suitable uses include
automobile trunks during the manufacture and shipping stages,
aircraft cargo holds, and the like. Label 10 can also be applied
directly to the surface of a packaging container or an article
without extending across two components movable relative to one
another. For example, label 10 can be applied to communications
equipment; computers, including peripheral equipment and
accessories; electronics; industrial products; retail products;
automobiles; and goods or packages being transported or shipped.
These examples of applications are meant to be illustrative and not
limiting.
The microporous film 12 can be any suitable microporous, diffusely
light-reflective film which can be made increasingly translucent or
transparent by filling its voids with a material having an index of
refraction close to that of the material of the film. The
microporous film 12 is preferably opaque, or nearly so, prior to
filling its voids with an appropriate transparentizing
material.
The microporous film 12 is initially opaque, or nearly so, because
it is diffusely-reflective when its voids are filled with a
material which has an index of refraction sufficiently different
from the index of refraction of film material. Typically, the voids
are initially filled with air. When a suitable transparentizing
material, typically a liquid, is applied to the microporous film
12, it fills the voids of the microporous film 12, thereby
displacing air from the voids. The transparentizing material and
the material of the microporous film 12 are chosen to have
sufficiently similar indices of refraction such that the
microporous film 12 becomes increasingly light transmissive as the
liquid substantially fills the voids of the film. If the indices of
refraction of the transparentizing liquid and the material of the
microporous layer 12 are sufficiently similar, the microporous film
12 can become substantially transparent as the transparentizing
liquid fills the pores.
In one preferred embodiment, label 10 includes first indicia, or
authenticating indicia, 18 and second indicia, or tamper indicating
indicia, 20 on the first surface 14 of microporous film 12. The
first and second indicia are normally not visually perceptible
through the microporous film 12 when the microporous film is in its
opaque state. Label 10 can optionally include third indicia 22 on
second major surface 16 of the microporous fill 12. Third indicia
22 will be permanently perceptible because it is on the exposed
surface of the label 10. This is seen in FIG. 3, in which first and
second indicia 18 and 20 are not visually perceptible through the
label 10 when looking at second major surface 16, and in which
third indicia 22 is visually perceptible. For clarity, third
indicia 22 is simply illustrated as the letter "C". However, it
should be understood that third indicia 22 can comprise any
suitable pattern, such as a bar code, company logo, alpha-numeric
characters such as a product identification, or the like.
First indicia 18 is rendered visually perceptible (as is second
indicia 20) when a transparentizing liquid is applied to the label
10, thereby rendering the microporous film transparent, or
substantially so, as illustrated in FIG. 4. According, the label 10
is an authenticating label in that a user can cause the microporous
film 12 to become transparent, thereby allowing the user to see the
first indicia, or authenticating indicia 18. For clarity, first
indicia 18 is simply illustrated as the letter "A". However, it
should be understood that the first indicia 18 can preferably be
selected to be of any desired pattern, alpha-numeric characters,
logo, or the like, so as to provide an indication to the user that
the label is authentic, and not a substituted counterfeit label.
The transparentizing liquid for performing the authenticating
function should preferably be a volatile liquid, so as to evaporate
over time, thereby returning the microporous film 12 to its opaque
state, and again obscuring the first indicia 18 from view, as seen
in FIG. 3. Suitable materials for authenticating indicia 18 include
water insoluble inks when the transparentizing fluid for
authentication is water. Such water insoluble inks include Flexo
Poly Reflex Blue FA 20775, available from Flint Ink of Detroit
Mich. When trichloroethylene is the transparentizing liquid, first
indicia 18 can comprise Flexo Fluorescent Red FA-22838 or Flexo
466-U Brown RVN-24853, both available from Flint Ink.
Authenticating indicia 18 can also comprise a metallized indicia
which is resistant to the transparentizing liquid. Additionally, it
is possible that authenticating indicia 18 be on the article 28
itself, or on an additional substrate bonded to the first surface
14 of the film 12.
It is also possible to add microparticles to the microporous film
12 during fabrication. The particles can be viewed at sufficient
magnification (for example 100.times.) to provide an additional
indication the label 10 is authentic. The microparticles can
comprise, for example, microtaggant colored particles and colored
glass beads as is known in the art.
The second, or tamper-indicating indicia 20 provides a permanent
indication that the label 10 has been tampered with in some way.
Typical methods of tampering include attempts to remove
surreptitiously the label 10 from an article 28 and subsequently
reapply the label without causing apparent change or damage to the
label. This generally involves an attempt to reduce or release the
adhesive bond between the adhesive layer 24 and the article 28.
This can be done by applying solvents to the label in an attempt to
dissolve the adhesive 24, applying oil to the label 10 in an
attempt to render the adhesive non-tacky, and applying a high
temperature to the label in an attempt to render the adhesive 24
non-tacky. Each of these methods is often combined with an attempt
to mechanically pull or peel the label from the article 10.
The second indicia 20 should comprise a material which is soluble
in most solvents used to dissolve the adhesive 24. When a solvent
is then applied to the label, at least a portion of the second
indicia 20 will bleed through the microporous film 12, and become
permanently visually perceptible on the second major surface 16 of
the film. When the solvent is initially applied, it may
transparentize the microporous film 12, as described above with
respect to the authenticating function illustrated in FIG. 4. When
the solvent evaporates, the microporous film will again become
opaque. However, the portion of the second indicia which has bled
through to the second major surface 16 will remain permanently
visually perceptible, as illustrated in FIG. 5. The second indicia
should be selected however, so as not to be soluble in the
transparentizing liquid chosen for the authenticating function
described above. The authenticating function should be performed so
as to render first indicia 18 temporarily visually perceptible,
without causing the second indicia 20 to bleed through to the
second major surface 16.
Second indicia 20 may be chosen to be soluble in most oils and the
like which are used to attempt to render the adhesive 24 non-tacky.
In case of such tampering, the second indicia 20 will bleed through
to the first major surface 16 as described above, thereby providing
a permanent indication of tampering. As an additional or
alternative indication to tempering with oil, oils are generally
non-volatile, and will therefore not evaporate from the label with
time. This will render the microporous film permanently
transparent. Accordingly, the second, or tamper-indicating, indicia
20 will provide a permanent, visually perceptible indication of
tampering for as long as the oil remains in the pores, whether or
not the second indicia 20 was soluble in the oil.
Suitable materials for second indicia 20 include solvent red #027
or solvent blue #027 dye powder from Spectra of Kearney, N.J. These
dyes are soluble in ketones, alcohols, and hydrocarbons that are
typically used in an attempt to defeat adhesives. This dye is not
soluble in water, and is suitable for use when the first indicia is
water insoluble, and water is to be used as the transparentizing
liquid. Other suitable materials for second indicia 20 include
Flexo Fluorescent Red FA-22838 or Flexo 466-U Brown RVN-24853, both
available from Flint Ink, and Macrolex Blue R (solvent blue #97)
from Verona Corporation. These inks are soluble in methanol,
acetone, and methyl ethyl ketone, which may be used in an attempt
to remove surreptitiously the label 10. These inks are not soluble
in trichloroethylene, and are therefore suitable for use as the
second indicia when the transparentizing fluid is to be
trichloroethylene.
When a high temperature is applied to the tamper indicating label
10 in an attempt to remove the adhesive 24 from the article 28,
this will cause the voids in the microporous film to collapse. This
will cause the microporous film 12 to become permanently
transparent. In this case, the second indicia 20 will be
permanently visually perceptible, providing an indication that the
label 10 has been tampered with.
For clarity, second indicia 20 is simply illustrated as the letter
"B". However, it should be understood that the second indicia 20
can be chosen to be any pattern, alpha-numeric combination, or
logo, to provide a user with an indication that the label 10 has
been tampered with. Second indicia 20 is preferably a message such
as "VOID" or "TAMPERED", or a pattern such as diagonal slashes or
the like which can optionally overlap the first indicia 18.
Alternatively, second indicia 20 can be mixed with adhesive 24.
As illustrated herein, one preferred embodiment of label 10
includes separate and distinct first and second indicia 18 and 20.
Providing distinct indicia allows for the first indicia to be a
difficult to copy pattern or logo, or alpha-numeric combination,
that will provide a user an indication that the label 10 is not
counterfeit and allows for the second indicia to be an obvious
indication of tampering, such as the word "VOID" or some suitable
pattern that will overlap or obscure the first indicia. It is also
within the scope of the invention that the authenticating and
tamper-indicating indicia be a single, or combined, indicia. In
such an a label, this combined indicia must be insoluble in the
transparentizing fluid while being soluble to most solvents
typically used in attempts to remove a label. The configuration of
such a combined indicia can be chosen to provide an indication of
authenticity upon rendering the label temporarily transparent, and
a permanent indication of tampering when the user observes that the
indicia remains visible for an extended period of time.
As stated above, attempts to defeat the adhesive 24 are often
combined with a mechanical pulling or peeling of the label 10 to
remove it from the article 26. Accordingly, the microporous film 12
can preferably be chosen to either substantially stretch and deform
when pulled from the article or to be brittle so as to break or
tear when pulled from the article. Either of these characteristics
will provide a permanent indication of tampering.
When it is desired to have a label 10 that will substantially
stretch and deform upon an attempt to remove the label from the
article 28, the material of the microporous film 12 should be
chosen to have substantial elongation at tensile failure. The
tensile strength of the film is preferably less than or
approximately equal to the strength of the bond between the label
10 and the article 28, such that the label will undergo substantial
plastic deformation and tensile failure before the adhesive bond is
broken. The tensile strength can instead be somewhat greater than
the bond strength, provided the label is substantially deformed
before the adhesive bond is broken. One preferred material for a
microporous film which will substantially stretch and deform is
polyethylene. Such films preferably have a tensile strength of
approximately 800-1200 psi and a percent strain at tensile failure
of between approximately 100-250%. Such measurements are taken in
accordance with ASTM D 882-83, using a 5 inch per minute pull and 1
inch wide samples. It should be understood that such values are
exemplary and non-limiting, and can be varied to obtain the desired
results, keeping in mind the particular adhesive 24 employed, the
surface of the article 28 to which the label will be adhered, and
the strength of the bond between the label and the article, both
tampered and untampered.
When it is desired to use a more brittle label 10 that will break
or tear upon attempt to remove it from the article 28, the material
of the microporous film 12 should be chosen to have a tensile
strength less than the strength of the bond between the label 10
and the article 28, and to have relatively low percent elongation
at tensile failure. For such an application, it has been found that
polypropylene microporous films and ethyl vinyl alcohol copolymer
(EVOH) microporous films are suitable. Suitable polypropylene flims
preferably have a tensile strength of approximately 300-800 psi and
a percent strain at tensile failure of approximately 5-15%.
Suitable EVOH films preferably have a strength of approximately
2500-3500 psi and a percent strain at tensile failure of
approximately 20-30%. As with the stretchable films discussed
above, it should be understood that such values are exemplary and
non-limiting, and can be varied to obtain the desired results,
keeping in mind the particular adhesive 24 employed, the surface of
the article 28 to which the label will be adhered, and the strength
of the bond between the label and the article, both tampered and
untampered.
Preferred microporous films include those described in U.S. Pat.
No. 4,539,256, "Microporous Sheet Material, Method of Making and
Articles Made Therewith," (Shipman), the entire disclosure of which
is incorporated herein by reference. Crystallizable polymers
suitable for use in the preparation of such microporous films are
well known and readily commercially available. The useful polymers
are melt processable under conventional processing conditions. That
is, on heating they will easily soften and/or melt to permit
processing in conventional equipment such as an extruder to form a
sheet. Crystallizable polymers, upon cooling their melt under
controlled conditions, spontaneously form geometrically regular and
ordered chemical structures. Preferred crystallizable polymers have
a high degree of crystallinity.
Particularly preferred are polyethylene microporous films and
polypropylene microporous films made generally in accordance with
the teachings of Shipman in which the blending compound has been
removed from the microporous film to provide a film comprising the
polymeric material itself. It is also possible to employ a film in
which the blending compound has not been removed, in which case the
transparentizing liquid must be chosen so as to be able to fill the
voids of the film despite the presence of the blending
compound.
The process of forming microporous films according to Shipman
generally includes:
(a) melt blending to form a solution comprising 30 to 80 parts by
weight of crystallizable thermoplastic polymer with 70 to 20 parts
by weight of compound with which the thermoplastic polymer is
miscible and in which the thermoplastic polymer will dissolve at
the melting temperature of the thermoplastic polymer and from which
the thermoplastic polymer will phase separate on cooling to a
temperature at or below the crystallization temperature of the
thermoplastic polymer;
(b) forming a film of the melt blended solution;
(c) cooling the film to a temperature at which the thermoplastic
polymer crystallizes to cause phase separation to occur between the
thermoplastic polymer and the compound thereby to provide an
article comprising an aggregate of a first phase comprising
particles of crystallized thermoplastic polymer in a second phase
comprising the compound with adjacent thermoplastic polymer
particles being distinct but having a plurality of zones of
continuity;
(d) orienting the film at least in one direction to separate
adjacent particles of crystallized thermoplastic polymer from one
another to provide a network of interconnected micropores
therebetween and to permanently attenuate the thermoplastic
polymeric material in the zones of continuity to form fibrils;
and
(e) optionally removing the compound from the film, e.g., by
solvent extraction.
The formed article, before orientation, is solid and generally
transparent comprising an aggregate of a first phase of particles
of crystallized thermoplastic polymer in a second phase of the
additive compound. The particles may be described as spherulites
and aggregates of spherulites of the polymer with additive compound
occupying the space between particles. Adjacent particles of
polymer are distinct but they have a plurality of zones of
continuity. That is, the polymer particles are generally surrounded
or coated by the compound but not completely. There are areas of
contact between adjacent polymer particles where phase separation
has not occurred and there is a continuum of polymer from one
particle to the next adjacent particle in such zones of
continuity.
On orienting or stretching, the polymer particles are pulled apart,
permanently attenuating the polymer in the zones of continuity
thereby forming the fibrils and forming minute voids between coated
particles, creating a network of interconnected micropores, thereby
rendering the article permanently translucent. On orienting or
stretching, the compound remains coated on or surrounds, at least
partially, the surfaces of the resultant thermoplastic polymer
particles. The degree of coating, of course, depends upon the
affinity of the compound for the surface of the polymer particle,
whether the compound is a liquid or solid, whether orientation
dislodges or disrupts the coating and on other factors which may be
relevant. The particles are usually at least partially coated after
orientation. Substantially all of the particles appear to be
connected by fibrils. The size of the micropores is easily
controlled by varying the degree of stretching, percent of additive
compound, melt-quench conditions, compound removal and
heat-stabilization procedures. The fibrils for the most part do not
appear to be broken by stretching but they are permanently
stretched beyond their elastic limit so that they do not
elastically recover to their original position when the stretching
force is released. As used herein, "orienting" means such
stretching beyond the elastic limit so as to introduce permanent
set or elongation of the article.
When fabricated as a film, the resulting article has a microporous
structure characterized by a multiplicity of spaced (i.e. separated
from one another), randomly dispersed, non-uniform shaped, equiaxed
particles of thermoplastic polymer coated with the compound and
connected by fibrils. (Equiaxed means having approximately equal
dimensions in all directions.)
The melting and crystallization temperature of a thermoplastic
polymer, in the presence of a blending compound or additive, is
influenced by both an equilibrium and dynamic effect. At
equilibrium between liquid and crystalline polymer, thermodynamics
requires that the chemical potentials of the polymer repeating unit
in the two phases must be equal. The temperature at which this
condition is satisfied is referred to as the melting temperature,
which will depend on the composition of the liquid phase. The
presence of impurities (such as a blending compound or additive) in
the liquid phase will lower the chemical potential of the polymer
repeating units in that phase. Therefore, a lower melting
temperature is required to re-establish the condition of
equilibrium, resulting in what is known as a melting temperature
depression.
The crystallization temperature and melting temperature are
equivalent at equilibrium. However, at non-equilibrium conditions,
which are normally the case, the crystallization temperature and
melting temperature are dependent on cooling and heating rate
respectively. Consequently, the terms "melting temperature" and
"crystallization temperature", when used herein, are intended to
include the equilibrium effect of the blending compound or additive
as well as the dynamic effect of the rate of heating or
cooling.
Compounds suitable for blending with the crystallizable polymer to
make the microporous materials taught by Shipman are liquids or
solids at room temperature and in which the crystallizable polymer
will dissolve to form a solution at the melting temperature of the
crystallizable polymer but will phase separate on cooling at or
below the crystallization temperature of the crystallizable
polymer. Preferably, these compounds have a boiling point at
atmospheric pressure at least as high as the melting temperature of
the crystallizable polymer. Compounds having lower boiling points
may be used in those instances where superatmospheric pressure may
be employed to elevate the boiling point of the compound to a
temperature at least as high as the melting temperature of the
crystallizable polymer. Generally, suitable compounds have a
solubility parameter and a hydrogen bonding parameter within a few
units of the values of these parameters for the crystallizable
polymer.
Some examples of blends of crystallizable polymers and blending
compounds which are useful in preparing microporous films suitable
for use in the present invention include: polypropylene with
mineral oil, dioctyl phthalate or mineral spirits;
polypropylene-polyethylene copolymer with mineral oil; and
polyethylene with mineral oil or mineral spirits.
A particular combination of polymer and blending compound may
include more than one polymer, i.e., a mixture of two or more
polymers, and/or more than one blending compound. Mineral oil and
mineral spirits are examples of mixtures of blending compounds
since they are typically blends of hydrocarbon liquids. Blends of
liquids and solids may also serve as the blending compound. It
should be understood that the polymer may include blended therein
certain conventional additive materials in limited quantity so as
not to interfere with the formation of the microporous material and
so as not to result in unwanted exuding of the additive. Such
additives may include antistatic materials, dyes, plasticizers, UV
absorbers, nucleating agents and the like. The amount of additive
is typically less than 10% of the weight of the polymer component,
preferably less than 2% by weight.
The melt is prepared by mixing 30 to 80% by weight of
crystallizable thermoplastic polymer and 70 to 20% by weight of
blending compound and heating the mixture to at least the melting
temperature of the crystallizable polymer. For ease of handling the
melt and in its casting, it is convenient to initiate the formation
of the melt-blend solution by heating the mixture at a temperature
in a range of about 25.degree. C. to about 100.degree. C. above the
melting temperature of the crystallizable polymer.
Microporous films are prepared by casting a sheet or layer of a
melt-blend solution comprising a crystallizable polymer and a
blending compound and, depending on the particular polymer, cooling
the article in a quench bath at an appropriate temperature
preferably between about 60.degree. C. and 225.degree. C. below the
equilibrium melting point of the pure crystallizable polymer to
provide a proper cooling rate. (For example for pure polypropylene
having an equilibrium melting point of 176.degree. C., a cooling
temperature between about 40.degree. C. and 60.degree. C. is
employed.) Some minor degree of experimentation may be required to
identify the appropriate temperature range which produces optimum
phase separation for a particular polymer-blending compound system,
this being well within the capability of one skilled in the art
once apprised of the present disclosure.
The attainment of the desired microporous materials is dependent on
the rate of cooling. During cooling, heat is removed from the
melt-blend solution until the crystallization temperature of the
crystallizable polymer is reached and solidification and phase
separation of the polymer can begin. Cooling temperatures greater
than about 225.degree. C. below the equilibrium melting point of
the pure crystallizable polymer cause too rapid quenching of the
melt and can result in single phase films which, although strong,
are substantially incapable of being rendered uniformly microporous
by stretching. Cooling at temperatures of less than about
60.degree. C. below the equilibrium melting point of the pure
crystallizable polymer allows for too slow a phase separation
(crystallization) of the crystallizable polymer with formation of
large spherulites that results in articles which are weak. Casting
articles or films which are too thick, i.e., thicker than 1,000
micrometers, to achieve the proper cooling rate as described above
also allows for too slow a crystallization. These parameters can be
varied to achieve a microporous film of desired tensile strength
and percent elongation.
The melt-blend solution is obtained by feeding into the hopper of
an extruder a mixture of crystallizable polymer and blending
compound through the extruder which is heated, preferably with 3
zones heated at increasing temperatures toward the extruder exit to
a slot die that has a slit gap of 25 to 1,000 micrometers.
Preferably, the crystallizable polymer is fed into the hopper of
the extruder and the blending compound is fed by a suitable device
into the extruder via a port in the extruder wall intermediate the
hopper and the extruder exit. It is also preferable to utilize a
suitable mixing device such as a static mixer between the extruder
exit and the slot die. In passing through the extruder, and, where
used, the static mixer, the mixture of polymer and blending
compound is heated to a temperature at or at least about 25.degree.
C. above the melting temperature of the crystallizable polymer (but
below the thermal degradation temperature of the polymer) and mixed
to form a melt-blend solution that is extruded through the slot die
as a layer into a liquid quench bath maintained at a suitable
temperature below the crystallization temperature of the
crystallizable polymer using a suitable coolant, e.g., water. In
some instances, e.g., for nylon and polyester, it is necessary to
cool the solution in the extruder before it reaches the die to a
temperature as much as 50.degree. C. cooler than the equilibrium
melting point of the pure polymer in order to obtain phase
separation in the sheet. The cooled film is then led from the
quench bath, to machine-direction orienting device, transverse
direction orienting device and then to take-up roller. Orienting in
two directions is, of course, optional.
The film is rendered microporous by stretching until micropores are
permanently developed or formed, typically to obtain at least an
increase in length (elongation) of about 10%. Stretching to provide
an elongation of about 10% to about 1,000% is typical. The actual
amount of stretching required will depend upon the particular
composition of the article and the degree of porosity desired.
Stretching may be provided by any suitable device which can provide
stretching in at least one direction and may provide stretching
both in that direction and in the transverse direction. Stretching
should be uniform to obtain uniform and controlled porosity.
Stretching in one direction typically causes the film to narrow or
"neck" in the cross direction so stretching a film to provide an
elongation of 50%, for example, does not produce a 50% increase in
the surface area of the film.
The microporous films are preferably dimensionally stabilized
according to conventional well known techniques by heating the
oriented sheet while it is restrained at a heat stabilizing
temperature.
The resulting microporous film is composed of randomly dispersed,
equiaxed, irregularly shaped particles of thermoplastic polymer.
Particles are spaced from one another to provide a network of
micropores therebetween. Particles are connected to each other by
fibrils which radiate from each particle to the adjacent
particles.
The blending compound may be removed from the microporous film to
provide a microporous film formed of the polymeric material per se.
Removal may be by solvent extraction, volatilization, or any other
convenient method. This removal may be preferable to facilitate the
application of the transparentizing liquid to perform the
authenticating function described above.
Another useful microporous film is a microporous ethylene-vinyl
alcohol copolymer (EVOH) film. The copolymer preferably has an
internal structure of a multiplicity of spaced, randomly dispersed,
non-uniform shaped particles of ethylene-vinyl alcohol copolymer.
Preferably, the particles are joined together by fibrils. The EVOH
film can be prepared by (a) melt blending a mixture of
ethylene-vinyl alcohol copolymer and a compatible polymer or
compound in which the copolymer will dissolve to form a solution at
the melting temperature of the copolymer, but will phase separate
on cooling at or below the crystallization temperature of the
ethylene-vinyl alcohol copolymer, (b) forming a film of the melt
blended mixture; (c) cooling the film to a temperature to cause
phase separation to occur between the compatible copolymer or
compound and the ethylene-vinyl alcohol copolymer, thereby
providing a film which is an aggregate of a first phase comprising
particles of ethylene-vinyl alcohol copolymer in a second phase of
the compatible polymer or compound with adjacent ethylene-vinyl
alcohol copolymer particles being distinct but having a plurality
of zones of continuity; and (d) i) collecting the film which has
adjacent ethylene-vinyl alcohol copolymer particles which form a
network of interconnected micropores therebetween, or ii)
extracting the compatible polymer or compound with water and
stretching the film in at least one direction while wet to further
separate adjacent ethylene-vinyl alcohol copolymer particles from
one another and to permanently attenuate the ethylene-vinyl alcohol
copolymer in the zones of continuity to form fibrils therebetween
and drying, or iii) extracting the compatible polymer or compound
with an organic solvent. One preferred commercial source of the
ethylene-vinyl alcohol copolymer is Eval Company of America.
When the compatible polymer or compound is removed from the film by
extraction with an organic solvent, the film may optionally be
stretched in a heated condition before or after removal of the
compatible polymer or compound to further separate adjacent
ethylene-vinyl alcohol copolymer particles from one another and to
permanently attenuate the ethylene-vinyl alcohol copolymer in the
zones of continuity to form fibrils therebetween. Preferably, the
compatible polymer or compound is a polyhydric alcohol.
The thus-formed EVOH microporous film is inherently hydrophilic,
can have porosity in the range of about 10 to 75 percent, and
possesses good tensile properties.
The formed film, before removal of the compatible polymer or
compound and stretching, is generally transparent or translucent,
and comprises an aggregate of a first phase of particles of EVOH in
a second phase of the compatible polymer or compound. The particles
may be described as spherulites and aggregates of spherulites of
the polymer, with the compatible polymer or compound occupying the
space between particles as described with regard to Shipman,
supra.
On stretching, the copolymer particles are pulled apart,
permanently attenuating the copolymer in zones of continuity,
thereby forming the fibrils and creating a network of
interconnected micropores. Such permanent attenuation also renders
the film permanently translucent or opaque. Also on stretching, if
the compatible polymer or compound is not removed, the compatible
polymer or compound remains coated on or surrounds, at least
partially, the surfaces of the resultant EVOH particles.
Substantially all of the particles appear to be connected by
fibrils.
The size of the micropores is easily controlled by varying the
degree of stretching, the amount of compatible polymer or compound
employed, melt-quench conditions, compatible polymer or compound
removal, and heat stabilization procedures. For the most part, the
fibrils do not appear to be broken by stretching, but they are
permanently stretched beyond their elastic limit so that they do
not elastically recover to their original position when the
stretching force is released. As used herein, "stretching" means
such stretching beyond the elastic limit so as to introduce
permanent set or elongation of the film.
The ethylene-vinyl alcohol copolymers may be random, block or graft
copolymers preferably having an ethylene unit content of about 20
to 90 mol percent, more preferably about 25 to 50 mol percent. The
degree of saponification of the copolymer is preferably at least
about 80 mol percent, more preferably at least about 95 mol
percent, based on the vinyl alcohol units in the copolymer. An
insufficient degree of saponification may result in insufficient
wet mechanical properties for some purposes. Preferably, the number
average molecular weight of the ethylene-vinyl alcohol copolymer is
at least about 2000 g/mol, more preferably at least about 20,000
g/mol, most preferably at least about 40,000 g/mol with higher
molecular weight copolymers frequently exhibiting better processing
and strength characteristics.
The ethylene and vinyl alcohol monomers may be copolymerized with
another copolymerizable monomer such as methacrylic acid, vinyl
chloride, methyl methacrylate, acrylonitrile and vinyl pyrrolidone
to form a terpolymer with the amount of copolymerizable monomer
being less than about 15 mol percent. Where a functional group is
present as a result of the copolymerizable monomer in the
terpolymer, the functional group can serve to provide sites for
crosslinking. Crosslinkages may be introduced into the
ethylene-vinyl alcohol copolymers or terpolymers by irradiation
such as, for example, ultraviolet, electron beam or gamma
irradiation. Crosslinkages may also be introduced into the
ethylene-vinyl alcohol copolymers before or after membrane
formation by treatment with an inorganic crosslinking agent such as
a boron compound or an organic crosslinking agent such as a
diisocyanate or a dialdehyde. Such crosslinking agents also may be
included in the melt blend as long as the phase separation is not
adversely affected. Useful ethylene-vinyl alcohol copolymers also
include those in which the functional hydroxyl groups of vinyl
alcohol units are partially crosslinked with an aldehyde such as
formaldehyde, acetaldehyde, butyraldehyde or benzaldehyde but
preferably to an extent of not more than about 30 mol percent.
Compounds suitable for blending with the ethylene-vinyl alcohol
copolymer to make the microporous materials are liquids or solids
at room temperature, in which the copolymer will dissolve to form a
solution at the melting temperature of the copolymer, but will
phase separate on cooling at or below the crystallization
temperature of the copolymer. Preferably, these compounds have a
boiling point at atmospheric pressure at least as high as the
melting temperature of the ethylene-vinyl alcohol copolymer.
However, compounds having lower boiling points may be used in those
instances where superatmospheric pressure may be employed to
elevate the boiling point of the compound to a temperature at least
as high as the melting temperature of the crystallizable polymer.
Generally, suitable compounds have a solubility parameter and a
hydrogen bonding parameter within a few units of the values of
these same parameters for the crystallizable polymer.
Suitable compounds for blending with the ethylene-vinyl alcohol
copolymers include, for example, polyhydric alcohols such as
polyethylene glycol, polypropylene glycol, ethylene glycol,
triethylene glycol, and glycerin. Preferably, the polyhydric
alcohol has a number average molecular weight below about 600 g/mol
to provide sufficient compatibility with the ethylene-vinyl alcohol
copolymer.
A particular combination of copolymer and compatible polymer or
compound may include more than one copolymer, i.e., a blend of two
or more copolymers and/or more than one compatible polymer or
compound. Optionally, if desired, the copolymer may include blended
therein certain conventional additive materials, which materials
should be limited in quantity so as not to interfere with the
formation of the microporous material and so as not to result in
unwanted exuding of the additive. Such additives may include, for
example, antistatic materials, antioxidants, dyes, plasticizers,
elastomers, ultraviolet light absorbers, and the like. When used,
the amount of such conventional additive is typically less than
about 10% of the weight of the copolymer component, and preferably
less than about 2% by weight.
The melt blend preferably comprises from about 30 to about 80 parts
by weight of the ethylene-vinyl alcohol copolymer and about 20 to
about 70 parts by weight of the compatible polymer or compound,
more preferably from about 40 to about 70 parts by weight of the
ethylene-vinyl alcohol copolymer and about 30 to about 60 parts by
weight of the compatible polymer or compound. The ethylene-vinyl
alcohol copolymer and the compatible polymer or compound are
preferably heated to at least the melting temperature of the melt
blend and preferably to a temperature in the range of about
10.degree. to 100.degree. C. above the melt temperature of the melt
blend for ease of handling of the melt.
The EVOH films useful in the present invention may be prepared by
casting a shaped film such as a sheet or layer from the melt blend
comprising the ethylene-vinyl alcohol copolymer and the compatible
polymer or compound. Depending upon the particular copolymer
employed, the film is cooled in a quench bath at an appropriate
temperature, preferably at least about 60.degree. C. below the
equilibrium melting point of the pure copolymer, to provide a
proper cooling rate. For example, for ethylene-vinyl alcohol
copolymer melt blend having an equilibrium melting point of
175.degree. C., a cooling temperature between about 40.degree. C.
and about 120.degree. C. or more is employed. Some minor degree of
experimentation may be required to identify the appropriate
temperature range which produces optimum phase separation for a
particular copolymer/compatible polymer or compound system, this
being well within the capability of those skilled in the art once
apprised of the present disclosure.
The attainment of the EVOH microporous films is dependent upon the
rate of cooling. During cooling, heat is removed from the melt
blend until the crystallization temperature of the ethylene-vinyl
alcohol copolymer in the melt blended mixture is reached, and
solidification and phase separation of the copolymer can begin.
Cooling temperatures greater than about 160.degree. C. below the
equilibrium melting point of the pure copolymer may cause too rapid
quenching of the melt blend and can result in single phase form,
which, although strong and transparent, are substantially incapable
of being rendered uniformly microporous by stretching.
The EVOH microporous membranes or films can be prepared using the
procedure described in Shipman, supra, with regard to FIG. 1 of
Shipman. The cooled film may then be led from the quench wheel to a
compatible polymer or compound removal bath containing water or
another solvent capable of dissolving the compatible polymer or
compound but not the ethylene-vinyl alcohol copolymer, if removal
of the compatible polymer or compound is desired. The film, when
water is used as the extraction bath, preferably is then directed
to a machine-direction stretching device and a transverse direction
stretching device, and then to a take-up roller for winding into a
roll. Where a solvent other than water is used to remove the
compatible polymer or compound, the film may be dried prior to
stretching and then stretched in a heated condition, e.g., from
about 70.degree. C. to 140.degree. C. Stretching in two directions
as done by the apparatus of FIG. 1 of Shipman is, of course,
optional.
A further method of forming a membrane material from the blended
melt includes casting the extruded melt onto a patterned chill roll
to provide areas where the blend does not contact the chill roll to
provide a membrane of substantially uniform thickness having a
patterned surface, the patterned surface providing substantially
skinless areas having high microporosity and skinned areas of
reduced microporosity. Such a method is described in U.S. Pat. No.
5,120,594 (Mrozinski) which is incorporated by reference herein.
Alternatively, the membrane material may be cast onto a wheel
coated with the compatible polymer or compound used to form the
melt blend. The membrane material can then be extracted and
oriented, i.e., stretched, as described above.
The porosity of the microporous films can be increased by
stretching, typically with an increase in length (elongation) of
about 10%. Stretching to provide a total area increase of about 10%
to about 1200% is generally useful. The actual amount of stretching
required will depend upon the particular composition of the film
and the degree of porosity desired. Stretching may be provided by
any suitable device which can provide stretching in at least one
direction, and may provide stretching both in that direction and in
the transverse direction. Stretching should be uniform to obtain
uniform and controlled porosity.
The microporous sheet materials of the present invention are
preferably dimensionally stabilized according to conventional, well
known techniques, such as by heating the stretched sheet, while it
is restrained, at a heat stabilizing temperature.
If the microporous film of the invention is dried but not annealed,
i.e., subjected to a heat treatment, it is dimensionally stable as
long as the film is kept in equilibrium by constant temperature and
humidity. Dimensional stability under conditions of changing
temperature and humidity can be improved with higher annealing
temperatures. Shrinkage when exposed to heat or water is a feature
which can make the microporous film useful for moisture indicating
or for forcing another compound from the pores of the film after
exposure. For some applications, it may be preferable to avoid
curling caused by mismatched dimensional changes of the microporous
film and optional release liner under heat and humidity. This can
be accomplished by selecting the release liner and microporous film
to have substantially equal expansivity properties.
The films of the present invention generally possess sufficient
tensile strength for subsequent handling, depending upon the
tensile strength of the ethylene-vinyl alcohol copolymer employed,
the zones of continuity, the extent of stretching, and the various
process conditions.
The compatible polymer or compound may be removed from the
microporous sheet to provide a unique microporous sheet material
formed only of the ethylene-vinyl alcohol copolymeric material.
Removal of the polyhydric alcohol may be accomplished by
extraction, volatilization, or any other convenient method. The
preferred extraction solvents include water and organic solvents
such as, for example, isopropyl alcohol, ethanol, methanol,
n-butanol, hydrochlorofluorocarbons, acetone, methyl ethyl ketone
and 1,1,1-trichloroethane.
The operation of the present invention will be further described
with regard to the following detailed examples. These examples are
offered to further illustrate the various specific and preferred
embodiments and techniques. It should be understood, however, that
many variations and modifications may be made while remaining
within the scope of the present invention.
EXAMPLES
In describing the examples, the following terms will be used. The
Gurley value is an indication of the porosity of the microporous
film 12, and is measured according to ASTM D-726-58 (1971), Method
A, "Standard Methods of Test for Resistance of Paper to Passage of
Air". The Gurley value is a measurement of the amount of time it
takes to force a known quantity of air through the film under
controlled conditions. Porosity is also indicated by a value
expressed as percent porosity, which is determined by dividing the
measured density of the polymeric microporous film by the bulk
density of the polymer, and subtracting that ratio from 1. Bubble
point is an indication of the diameter of the pores of the
microporous film, and is measured in accordance with ASTM F 316-86,
"Standard Test Methods for Pore Size Characteristics of Membrane
Filters by Bubble Point and Mean Flow Pore Test." This test
determines how much pressure is required to force a given volume of
gas through the microporous film after prewetting the film. This
time is used to calculate the flow-limiting diameter of the
pores.
Example 1
A brittle microporous film comprising a 0.1 mm (0.004 in) thick
film of EVOH having a Gurley value of 26.5 sec., a 0.66 micrometer
pore size as determined by the bubble point, and a porosity of
51.8% was used to fabricate the label. A 0.05 mm (0.002 in) thick
layer of pressure sensitive adhesive comprising an Acrylic 300
adhesive from Minnesota Mining and Manufacturing Company, St. Paul,
Minn. was used. First or authenticating indicia 18 comprising a
water insoluble ink commercially available as Flexo Poly Reflex
Blue FA 20775 from Flint Ink was printed on first major surface 14
of microporous film 12. Second, or tamper-indicating indicia 20 was
provided by incorporating 0.3% by weight red dye solvent powder 027
available from Spectra into the adhesive. Water was used to
transparentize the microporous film to reveal the authenticating
indicia, without causing the tamper-indicating indicia to bleed
through to second major surface 16 of the microporous film. When
the water evaporated, the film returned to its opaque state.
Toluene, methyl ethyl ketone, and methanol were each applied to the
label as would be done in an attempt to remove the adhesive bond to
a substrate. Each solvent caused the second indicia to bleed
through to the exposed surface of the label, providing a permanent
indication of tampering.
Example 2
Example 2 was prepared according to Example 1, except that solvent
blue 97 dye, available from Spectra was used as second indicia 20.
The label of Example 2 performed as reported with reference to
Example 1.
Example 3
A brittle microporous film comprising a 0.1 mm (0.004 in) thick
film of polypropylene with the oil removed from the pores, having a
Gurley value of 3 sec., a 1.24 micrometer pore size as determined
by the bubble point, and a porosity of 80% was used to fabricate
the label. A 0.05 mm (0.002 in) thick layer of pressure sensitive
adhesive comprising an Acrylic 100 adhesive from Minnesota Mining
and Manufacturing Company, St. Paul, Minn. was used. In this
embodiment, the same indicia performed as both the authenticating
indicia and the tamper-indicating indicia. Indicia 18 comprising an
ink commercially available as Flexo Fluorescent Red FA-22838 from
Flint Ink was printed on first major surface 14 of microporous film
12. Trichloroethylene was used to transparentize the microporous
film to reveal the authenticating indicia, without causing the
tamper-indicating indicia to bleed through to second major surface
16 of the microporous film. When the trichloroethylene evaporated,
the film returned to its opaque state. Methanol, acetone, and
methyl ethyl ketone solvents were each applied to the label and
each caused the indicia to bleed through to second surface 16 of
film 12, providing a permanent indication of tampering.
Example 4
Example 4 was prepared according to Example 3, except that Flexo
466-U Brown RVN-24853 from Flint Ink was used for the indicia. The
label of Example 4 performed as reported with Example 3.
Example 5
A stretchy microporous film comprising a 0.09 mm (0.0035 in) thick
film of polyethylene with the oil removed from the pores, having a
Gurley value of 18 sec., a 0.41 micrometer pore size as determined
by the bubble point, and a porosity of 77% was used to fabricate
the label. A 0.05 mm (0.002 in) thick layer of pressure sensitive
adhesive comprising an Acrylic 100 adhesive from Minnesota Mining
and Manufacturing Company, St. Paul, Minn. was used. The label of
Example 5 was prepared with indicia and tested with the same
results as reported with Example 3.
Example 6
A stretchy microporous film comprising a 0.025 mm (0.001 in) thick
film of polypropylene with the oil left in the pores, having a
Gurley value of 184 sec., a 0.29 micrometer pore size as determined
by the bubble point, and a porosity of 20% was used to fabricate
the label. A 0.05 mm (0.002 in) thick layer of pressure sensitive
adhesive comprising an Acrylic 100 adhesive from Minnesota Mining
and Manufacturing Company, St. Paul, Minn. was used. The label of
Example 6 was prepared with indicia and tested with the same
results as reported with Example 3.
The present invention has now been described with reference to
several embodiments thereof. The foregoing detailed description and
examples have been given for clarity of understanding only. No
unnecessary limitations are to be understood therefrom. It will be
apparent to those skilled in the art that many changes can be made
in the embodiments described without departing from the scope of
the invention. For example, although the label has been described
herein as having the indicia and adhesive applied directly to the
first major surface of the microporous film, it is understood that
an additional substrate can be bonded to the first surface of the
microporous film, and that the indicia can be on either or both of
the film and the substrate. Thus, the scope of the present
invention should not be limited to the exact details and structures
described herein, but rather by the structures described by the
language of the claims, and the equivalents of those
structures.
* * * * *